Drilling Out Main Jets

[kbruce6057]

Hello, rebuilding carbs, is it possible to drill out the main jets to 122.5? As you might guess I am running exhaust and filter mods. Just wondering if anyone has done it?

[Billy Ricks]

Jets are cheap. Don't screw around drilling them. I may have a set of 122.5s you can have.

[49er]

Hello, rebuilding carbs, is it possible to drill out the main jets to 122.5? As you might guess I am running exhaust and filter mods. Just wondering if anyone has done it?

Yes, it can be done but is not recommended unless you have plenty of past jetting experience. Most here will suggest you use a Dyna Jet kit for a very good reason. There is no guess work required and only minor fine tuning required.
You need to tell us what engine/carb combination you are running!
You also need to be comparing "apples with apples" when quoting jet numbers, either flow rate or hole diameter.

[Billy Ricks]

What he said. The most important part of a jet kit are the slotted needles. Just throwing in bigger mains won't help much, the rest of the rev range needs tuning too. You might be able to shim the needles up a little but get ready to pull the carbs lots of times to get it anywhere in the ballpark.

[tkent02]

How would you know what diameter drill bit to use?
Drill bits don't come calibrated in flow rates.

You might get one "a little bit bigger" but what good is that?

[Nessism]

I've drilled jets before, back when I was a poor student. Yea I know, it's not recommended but who cares? If you have a full drill kit, with both numbered and lettered sizes, it's a simple matter to go just a smig over where the orifice is currently. Slam the jet back in and go do some plug chops. I've even drilled pilots using a small pin as a drill bit; measure the diameter with a micrometer and work it in there. Again, not recommended but not much to loose. If it doesn't work, get some new jets.

[lecroy]

I drill my own jets because I have to. You would need to buy a drill kit and make a fixture for them.
I would call Sudco and be done with it.

[jeff.saunders]

Drilling jets will give you a starting point for tuning, but the quality will be suspect. The easiest way to illustrate this - take a set of factory Mikuni or Keihin jets and squirt WD40 through the jet (with the little red tube). Then do the same with a home drilled jets. The cone of fluid will be consistent with the factory jets Repeat with all the factory jets and you'll understand why you don't want to drill jets.

[kbruce6057]

Yes, it can be done but is not recommended unless you have plenty of past jetting experience. Most here will suggest you use a Dyna Jet kit for a very good reason. There is no guess work required and only minor fine tuning required.
You need to tell us what engine/carb combination you are running!
You also need to be comparing "apples with apples" when quoting jet numbers, either flow rate or hole diameter.

I have 1982 gs850 with cv carbs, vance & hines 4 into 1 and k&n pods.

Steve

[kbruce6057]

What he said. The most important part of a jet kit are the slotted needles. Just throwing in bigger mains won't help much, the rest of the rev range needs tuning too. You might be able to shim the needles up a little but get ready to pull the carbs lots of times to get it anywhere in the ballpark.

So if I can drill out the jets, all I need is the slotted needles? Can you buy them seperately? I just can't see $122 for a jet kit. How do you shim up the needles? I've got more time than money.

Thanks,
Steve

[Nessism]

So if I can drill out the jets, all I need is the slotted needles? Can you buy them seperately? I just can't see $122 for a jet kit. How do you shim up the needles? I've got more time than money.

Thanks,
Steve

The needle height is controlled by the thickness of the spacer on TOP of the needle clip. From the factory there is a plastic spacer, take it off and replace with a small stack of washers about 1/2 as thick. This is a good starting point from which to tune. Remove additional washers to raise the needle further.

Good luck.

[49er]

I have 1982 gs850 with cv carbs, vance & hines 4 into 1 and k&n pods.

Steve

Now you need a response from another member who has used this combination of mods, or somthing quite similar. They may supply the correct DJ kit # and the suggested needle positions.

You could end up spending far more money than the $122 cost of the jet kit, say on a good range of ACCURATELY sized drills, or worse still some holed pistons.

I have a comprehensive jet drill set. Each replacement drill I purchase is measured to varify the accuracy of its size. It is quite common for these smaller drills to be slightly undersized/oversized than what they are sold as!
Correctly sized drills can also be used to confirm that a PO hasn't previously drilled out a jet.
Even new OEM jets have been found to vary in size from what they are marked as.

[mac1170]

gday 49er, im trying to work out, when you remove the washers, from the needle, in the slide , doesnt that lean the mixture as it drops the needle further into the main jet?, regards.

[49er]

gday 49er, im trying to work out, when you remove the washers, from the needle, in the slide , doesnt that lean the mixture as it drops the needle further into the main jet?, regards.

Giday Mac, no it will richen the mixture by raising the needle. IMO, you shouldn't run the needle without the nylon washer as it acts as a damping mechanism when the throttle is released suddenly, especially on manual slide carbs. Cheers.

[Nicholaschase29]

There is a spring under the needle that always holds upwards force on the needle. There is also a piece that goes on top of the needle that the needle gets pushed up against from the force of the spring underneath. If you add washers to the top of the needle the needle will be lowered (and the spring will be more compressed) because the needle hits the piece that goes on top of the needle at a lower position because of the extra washer on it. I hope that makes some sense..

Nick